authPrompt: propagate gdm "reset" signal after user switching

After a user types in their password at the login screen, one
of two things can happen

1) a new session is started
2) an existing session is switched to

In the latter case, GDM sends a reset signal to the login screen,
so it knows to go back to the user list and wait to be summoned
again.

Unfortunately, all reset signals are ignored after verification
success.  The reason is because the reset handler was copied from
the unlock dialog as part of a deduplication effort in commit
7e7295f259febf34c89659a9bcb05f9924fa1976 and the unlock dialog
handler at the time also emitted a "failed" signal on reset
(which wouldn't make sense to emit after success).

These days "failed" is handled in a different way.

This commit changes the code to let reset signals through after
successful verification.

https://bugzilla.gnome.org/show_bug.cgi?id=710456